THIS invention relates to mills, and more particularly it relates to mills for milling ore or the like.
A common type of mill used in the mining industry for milling ore includes a drum structure mounted to rotate about a horizontal axis extending longitudinally through the bore of the drum structure. The drum structure usually includes an inlet end through which ore is fed into the structure, and an outlet end through which the milled ore is discharged. The outlet end often includes a screen which only allows ore fragments smaller than a predetermined size to be discharged from the drum structure. Because of the high abrasion forces present in the drum structure, the bore thereof is usually lined with an interchangeable liner. The liner prevents the drum mantle itself from becoming worn out as a result of the abrasion forces and when the lining is worn out it can be replaced by a new lining.
Most mills also include lifter bars mounted on the liner to extend into the drum structure. The lifter bars comprise elongate bars extending along the length of the bore of the drum structure as solid walls spaced apart from and substantially parallel to each other. Each elongate bar usually comprises interchangeable sections located next to each other along the length of the drum, each section being separately replaceable as it becomes worn out.
In use the drum structure of the mill is rotated, causing the lifter bars to catch and scoop up the mill contents in the bottom of the structure. The scooped up contents slide from the lifter bars when approaching the top of the mill and fall onto the contents still located in the bottom of the drum structure thereby crushing and milling the ore.
In the case of a pure autogenous mill the mill contents comprise only ore. In the case of a semi-autogenous mill the mill contents comprise ore and grinding media such as steel balls or the like. The grinding media enhances the milling action when, during rotation of the drum it is scooped up and dropped onto the ore in the bottom of the drum.
A disadvantage of the type of mill described above is that the lifter bars indiscriminately scoop up all sizes of ore fragments. Not only are the large ore fragments, which are more effective in the milling process scooped up and dropped onto the rest of the ore, but so are the small ore fragments which are less effective in the milling process. Energy is therefore wasted to some extent by scooping up and hoisting the small ore fragments. Another and closely related disadvantage is that the small ore fragments usually migrate through the larger ore fragments to the bottom of the drum structure and thus tend to fill up the lifter bars with the result that less of the larger ore fragments [and other grinding media in the case of a semi-autogenous mill] are scooped up by the lifter bars.